selftests: mptcp: add ADD_ADDR mibs check function

This patch added the ADD_ADDR related mibs counter check function
chk_add_nr(). This function check both ADD_ADDR and ADD_ADDR with
echo flag.

The output looks like this:

 07 unused signal address             syn[ ok ] - synack[ ok ] - ack[ ok ]
                                      add[ ok ] - echo  [ ok ]
 08 signal address                    syn[ ok ] - synack[ ok ] - ack[ ok ]
                                      add[ ok ] - echo  [ ok ]
 09 subflow and signal                syn[ ok ] - synack[ ok ] - ack[ ok ]
                                      add[ ok ] - echo  [ ok ]
 10 multiple subflows and signal      syn[ ok ] - synack[ ok ] - ack[ ok ]
                                      add[ ok ] - echo  [ ok ]
 11 remove subflow and signal         syn[ ok ] - synack[ ok ] - ack[ ok ]
                                      add[ ok ] - echo  [ ok ]

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>